Studies have shown that students, after having studied physics concepts and being familiar with them for an appreciable time, may nevertheless lack the ancillary knowledge needed to use such concepts reliably; correspondingly, they exhibit major misconceptions and errors. Provided in this paper is an analysis of the ancillary knowledge required to make a scientific concept of principle effectively usable. Property concepts are addressed since these are centrally important to descriptions needed in science. This analysis includes, as a subset, the ancillary knowledge for a simple entity concept. Furthermore, the ancillary knowledge of a property concept is essentially the same as that for a principle. The most important ancillary knowledge required to make a concept effectively usable is that required to interpret the concept appropriately. This analysis of the ancillary knowledge needed for concept interpretation points out some practical implications for the learning and teaching of scientific concepts/principles. Students could be made aware of the ancillary knowledge (focusing on specification of concept, concept values, independent variables; instantiation; and error prevention) required to interpret a particular concept of interest or for use as a general skill in effectively learning any newly encountered concept or principle. (JN)